368 BIOLOGY FOR BEGINNERS 



side come two premolars and three molars, all with rough flat 

 crowns and used to crush the food. The first or " milk " teeth 

 lack the premolars and one set of molars hence number but twenty 

 in all. The reason for having two sets is to allow for the growth 

 of the jaw. Hence, if the first teeth are allowed to decay and are 

 pulled too soon, the jaw never gets its proper shape and the later 

 teeth are crowded and irregular. At the proper times the roots of 

 the first teeth are absorbed and they make way easily for the 

 permanent teeth and the jaw is developed into proper shape. 



The numbers of teeth are often expressed in fractional form, 

 and are easily remembered in this way. Beginning at the front in 

 the middle of the jaw and putting the upper teeth above and the 

 lower teeth below, we have the " dental formula " for the adult 

 and first sets as follows-: 



Incisors Canines Premolars Molars 

 First set (20) ' f } ' jj | 



2123 

 Permanent set (32) 



L \ L o 



The last pair of molars may not appear till about the 20th year 

 and are therefore called the wisdom teeth, as one is supposed to 

 have acquired some wisdom by that time. 



Among other animals the teeth vary a great deal in size and 

 number, but there is none that has a greater variety of kinds. 

 Horses and cattle have molars greatly developed, cats and dogs 

 have canines long and sharp, while rats and squirrels develop the 

 incisors excessively for gnawing. Vegetable foods require broad 

 grinding teeth, animal food needs sharp canines and shear-cutting 

 premolars, while man, being adapted for a mixed diet, has all forms 

 moderately developed. Chewing is one of the mechanical processes 

 which prepares the food for chemical action by the digestive 

 fluids. 



Glands. Digestive fluids are secreted by organs called glands. 

 A gland consists of a group of cells adapted for producing a fluid 



